The present invention relates to magnetic disk digital data storage and retrieval systems and, more particularly, to such systems in which a magnetoresistive sensor is used in the retrieval of magnetically stored digital data.
Magnetic disk digital data storage and retrieval systems are used to store data in a moving magnetic media layer provided on a moving disk through use of a storage, or "write" electrical current-to-magnetic field transducer, or "head" positioned immediately adjacent thereto. The data is stored or written to the magnetic media by switching the direction of flow of a substantially constant magnitude write current which is established in conductive windings in the write transducer. Each write current direction transition results in a reversal of the magnetization direction in that portion of the magnetic media just passing by the transducer during the flow established in the new direction with respect to the magnetization direction in the media induced by the previous flow in the opposite direction.
When such stored data is to be recovered, a retrieval, or "read" magnetic field-to-voltage transducer, or "head", is positioned to have the magnetic media, containing this previously stored digital data, pass closely thereby such that flux reversal regions in that media create a time varying magnetic field which can be sensed to provide a corresponding output signal. One kind of sensor for that purpose is a magnetoresistive sensor, a sensor that is provided in an operating circuit so that such flux reversal magnetic field regions result in correspondingly changing a circuit parameter to provide voltage pulses for forming an output signal for this retrieval transducer.
Such magnetoresistive sensors can often be advantageously fabricated using ferromagnetic thin-film materials. These sensor devices so formed may be provided on a surface of a monolithic integrated circuit chip in some instances to make convenient the provision of electrical connections between the device and some or all of the operating circuitry therefor provided in the integrated circuit chip which chip is then mounted on a positioning arm. In other instances, such sensors are independently mounted on such a positioning arm to be positioned conveniently close to the moving magnetic material for retrieval, and an interconnection lead is provided between the sensor, or the sensor and some integrated circuitry, and the rest of the operating circuit formed in a monolithic integrated circuit chip.
Such a sensor mounted on such an arm in an operating circuit is typically desired to have one end thereof connected to an electrical voltage potential which is the same as that electrical voltage potential at which the moving magnetic media is operated, typically the circuitry ground, to prevent the occurrence of electrical discharge "arcing" between the sensor and the media over the very small distance therebetween. Another effect of the magnetoresistive sensor on the operating circuit in which it is provided is the addition of a significant electrical resistance in that circuit due to the sensor because such sensors must be made quite small so as to have dimensions on the order of the area taken up by digital data bit magnetization areas in the moving magnetic media. A further characteristic of such magnetoresistive sensors in the past has been the substantial variation in resistance from sensor to sensor as a result of the fabrication process therefor. In such circumstances, there was a desire to have the output signal from the operating circuitry for the magnetoresistive sensor represent the quantity .fwdarw.R/R, where .fwdarw.R is the signal change and R is the nominal sensor resistance, rather than just .fwdarw.R to provide some normalization in the output signal to counter such variations in the value of R resulting from the fabrication process.
These circuit desires have led in the past to the use of operating circuits which depend on circuit signal feedback loops to provide the desired operating circuit characteristics. However, the frequency content of signals to be sensed by a magnetoresistive sensor in a magnetic disk digital data storage and retrieval system can range from 20MHz to 200MHz or more. Over such a large frequency range, the use of feedback loop circuits raises questions of whether stability can be maintained in the circuit over this range in view of changing circuit parameters due to fabrication process variations and changing environmental conditions. Thus, there is a desire to avoid such stability problems in the operating circuitry for such magnetoresistive sensors while still providing substantial gain therefor and low noise operation therewith.